Sars International Centre for Marine Molecular Biology, University of Bergen, Bergen, Norway.
PLoS Biol. 2019 Jul 29;17(7):e3000408. doi: 10.1371/journal.pbio.3000408. eCollection 2019 Jul.
Most bilaterian animals excrete toxic metabolites through specialized organs, such as nephridia and kidneys, which share morphological and functional correspondences. In contrast, excretion in non-nephrozoans is largely unknown, and therefore the reconstruction of ancestral excretory mechanisms is problematic. Here, we investigated the excretory mode of members of the Xenacoelomorpha, the sister group to Nephrozoa, and Cnidaria, the sister group to Bilateria. By combining gene expression, inhibitor experiments, and exposure to varying environmental ammonia conditions, we show that both Xenacoelomorpha and Cnidaria are able to excrete across digestive-associated tissues. However, although the cnidarian Nematostella vectensis seems to use diffusion as its main excretory mode, the two xenacoelomorphs use both active transport and diffusion mechanisms. Based on these results, we propose that digestive-associated tissues functioned as excretory sites before the evolution of specialized organs in nephrozoans. We conclude that the emergence of a compact, multiple-layered bilaterian body plan necessitated the evolution of active transport mechanisms, which were later recruited into the specialized excretory organs.
大多数两侧对称动物通过专门的器官（如肾和肾管）来排泄有毒代谢物，这些器官在形态和功能上具有对应关系。相比之下，非肾动物的排泄方式在很大程度上是未知的，因此对祖先排泄机制的重建是有问题的。在这里，我们研究了 Xenacoelomorpha（与 Nephrozoa 是姐妹群）和刺胞动物（与 Bilateria 是姐妹群）成员的排泄模式。通过结合基因表达、抑制剂实验以及暴露于不同环境氨条件下的实验，我们表明 Xenacoelomorpha 和 Cnidaria 都能够通过与消化相关的组织进行排泄。然而，尽管刺胞动物 Nematostella vectensis 似乎以扩散为主要排泄方式，但两种 Xenacoelomorpha 则同时使用主动运输和扩散机制。基于这些结果，我们提出在 Nephrozoa 中专门器官进化之前，与消化相关的组织就已经起到了排泄部位的作用。我们的结论是，紧凑的、多层次的两侧对称体节动物身体计划的出现需要进化出主动运输机制，这些机制后来被招募到专门的排泄器官中。
